THE EARLIEST-KNOWN MUSTELID IN NORTH AMERICA AND THE CONVERGENT EVOLUTION OF SMALL BODY SIZE IN MUSTELIDAE

Until now, the pre-Miocene fossil record of mustelids in North America has been restricted to specimens attributable to oligobunine taxa and isolated mandibular and dental remains tentatively allocated to the genus Plesictis. The oligobunines are known exclusively from North America, and their relation to Eurasian taxa is not well understood. In the present study, we report on a complete cranium of Plesictis julieni (JODA 8167) from the Turtle Cove Member of the John Day Formation, between tuffs dated from 27.14 and 25.9 MA.

This specimen confirms the presence of Plesictis in the late Oligocene (Arikareen) of North America, and pushes back the known record of the genus (previously 24.4-23.7 Ma, set by Plesictis plesictis from France). Like the holotype specimen of Plesictis julieni, JODA-8167 lacks an alisphenoid canal and a postprotocrista on the M1 (synapomorphies of Mustelidae), but retains a dorsally deep suprameatal fossa (a feature occasionally suggested to be unique to Procyonidae). A combined phylogenetic analysis, using Bayesian Inference of molecular (five genes totalling 5490 bp) and morphological (100 characters) data, and a parsimony analysis of morphological data with a molecular backbone enforced, recover similar topologies. P. julieni is identified as a close relative of the oligobunines Zodiolestes and Promartes in a sister group to a neomustelid clade. Two other taxa traditionally allocated to Oligobuninae - Megalictis and Brachypsalis - are placed on the stem of the neomustelid divergence, thereby rendering Oligobuninae paraphyletic.

Plesictis julieni displays a skull size comparable to that of the least weasel (Mustela nivalis), the smallest extant carnivoran. It has been suggested selection for a smaller body size in mustelids was linked to aridification in the late Pliocene and Pleistocene, as burrowing rodents began to proliferate in the open grassland habitats. Previous work on inferring paleoprecipitation from paleosols from the John Day Formation suggested a drastic decrease in precipitation leading up to the early Arikareean, producing a semi-arid climate and open habitats. The first burrowing rodents (e.g., Palaeocastor, Pleurolicus) in Oregon appear at this time and many of these taxa co-occur with Plesictis julieni in the Turtle Cove Member.